OF RESIDENTIAL STRUCTURES by
Heather Beata Dye
A Dissertation Presented in Partial Fulfillment of the Requirements for the Degree
Doctor of Philosophy
ARIZONA STATE UNIVERSITY May 2008
© 2008 Heather Beata Dye All Rights Reserved
OF RESIDENTIAL STRUCTURES by
Heather Beata Dye
has been approved April 2008
Graduate Supervisory Committee: Sandra L. Houston, Co-Chair
Bruno D. Welfert, Co-Chair Claudia Zapata
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It is well established that damage to structures built on expansive soils is mainly caused by changes in soil suction. Suction changes are generally attributed to changes in environmental conditions such as change in water table depth, surface irrigation and landscape, resulting in changes in the surface and groundwater regime. Slabs-on-grade must resist both long-term and short-term moisture-change induced soil volume change. The design of residential structures in arid regions is especially challenging because the soil experiences large variations in matric suction and associated substantial volume change. As a result, a large number of houses experience minor to severe distress.
Unsaturated soil mechanics theory is used in the determination of unsaturated soil behavior. It is the purpose of this research work to help bridge the gap between theory and practice in the design of residential foundations on expansive soils. One part of this study relates to investigating the depth and degree of wetting associated with moisture flow through expansive soils through modeling and field studies in semi-arid region for typical residential construction development, as well as assessment of foundation performance. A number of steps were taken towards the goal of developing a better understanding of expansive soils behavior and field conditions leading to problems with expansive soils. These steps include: 1) numerical modeling of moisture flow through expansive soils in one- and two-dimensions. Two extreme surface flux conditions were considered, desert and excessively irrigated turf landscapes. The numerical results are applicable to regions with low to moderate expansion potential and Phoenix, Arizona environmental conditions. 2) Development of map illustration to identify locations with low to medium swell potential in the Phoenix Valley. 3) Comparisons of the numerical results to field evidence on depth of wetting and depth of active zone. 4) Evaluation of stability, convergence and numerical challenges for unsaturated moisture flow through expansive soils using Richards’ equation. Sources of numerical instabilities were identified and potential improvements discussed. 5) Survey of Arizona region practitioners to identify current design and construction practices, and 6) Analysis of forensic investigations to identify the nature and common causes of residential distress.
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This research work was made possible through the financial support by Homebuilders Association of Central Arizona (HBACA) and Construction Inspection and Testing Co. (CIT). The author is very grateful for the inspiration, encouragement and support of Dr. Sandra Houston and Dr. Bill Houston. I would like to thank Dr. Bruno Welfert for his time and unlimited patience in explaining numerical concepts applicable to the problem considered in this research work. The contribution of Dr. Claudia Zapata is also acknowledged, who, most importantly, was a friend in a time of need. Additionally, I would like to express my gratitude to practitioners, Brian Juedes (PE, Senior Vice President, Felten Group), Scott Neely (Terracon Inc.) and Dr. Kirby Meyer (PE, Chairman, MLAW), for partial review of this research work with calculations and edits. The development of map illustration was made possible through the collaboration with PhD students, Drew Lucio and Sonal Singhal. Their contribution is gratefully acknowledged. This work, in part, was made possible through the good will of numerous Arizona based companies which interviewed with ASU and/or released their geotechnical/forensic data for research purposes. Their contribution is gratefully acknowledged.
Finally, I would like to thank my husband Brian, who convinced me to continue with higher education, provided moral support and was a source of fascinating discussions about unsaturated soil mechanics and computer computations; my brother David, on whom I can always count on; and most importantly my father and my mother who instilled in me the appreciation for knowledge. Through their personal sacrifices I was able to immigrate to the United States and pursue my dreams.
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Page
LIST OF TABLES ... xvi
LIST OF FIGURES ... xix
NOMENCLATURE... xxviii
CHAPTER 1 INTRODUCTION... 1
1.1 Overview ... 1
1.2 Historical Background ... 3
1.3 Research Objective and Scope ... 6
1.4 Research Methodology ... 7
1.5 Outline of Report ... 9
1.6 Key Findings ... 11
2 LITERATURE REVIEW ... 13
2.1 Introduction ... 13
2.2 Factors Affecting Swell and Moisture Migration ... 14
2.3 Moisture Variation within Soil Profile ... 16
2.3.1 Infiltration and Wetting Front ... 16
2.3.2 Soil Profile ... 17
2.3.3 Definition of Active Zone Depth and Related Terms ... 19
2.3.3.1 Active Zone Depth ... 21
2.3.3.2 Zone of Seasonal Moisture Fluctuation ... 21
2.3.3.3 Depth of Wetting ... 21
2.3.3.4 Depth of Potential Heave ... 22
2.3.4 Edge Moisture Variation Distance ... 22
2.4 Causes of Water Content Change; Field Observations of Moisture Migration and Heave ... 23
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2.4.2 Seasonal Water Content Change ... 26
2.4.2.1 Field Studies of Seasonal Moisture Variations ... 28
2.4.2.2 Field Studies of Seasonal Temperature Variations ... 30
2.4.2.3 Field Studies of Monotonic vs. Seasonal Moisture Variation and Heave .. 31
2.4.3 Accidental Changes of Water Content ... 32
2.5 Soil Response to Change in Water Content ... 35
2.5.1 Settlement ... 36
2.5.2 Shrinkage ... 37
2.5.3 Heave ... 39
2.5.4 Fatigue of Swelling ... 39
2.6 Performance of Residential Construction ... 40
2.6.1 As-built Floor Deviation from Horizontal ... 41
2.6.2 Post-Construction Slab Distortion ... 42
2.7 Mitigation measures... 43
2.7.1 Removal, replacement and recompaction ... 43
2.7.2 Mechanical Stabilization ... 44
2.7.3 Chemical Stabilization ... 44
2.7.4 Stabilization of Water Content ... 45
2.7.4.1 Passive Stabilization ... 45
2.7.4.2 Active Stabilization ... 48
2.7.5 Site Drainage and Control of Landscape Watering ... 48
2.8 Classification of Swell Potential Based on Soil Properties ... 49
2.8.1 Mineralogical Classification ... 50
2.8.1.1 Cation Exchange Capacity ... 52
2.8.1.2 Cation Exchange Capacity and Soil Properties ... 53
2.8.1.3 Atterberg Limits ... 55
viii 2.8.2.1 Atterberg Limits ... 55 2.8.2.2 Linear Shrinkage ... 58 2.8.2.3 Colloid Content ... 58 2.8.2.4 Suction ... 59 2.8.3 Direct Measurement ... 62
2.9 Unsaturated Soil Mechanics Theory ... 64
2.9.1 Soil Suction and Soil Moisture ... 64
2.9.2 Measurement of Soil Suction ... 66
2.9.3 Soil Water Characteristic Curve ... 69
2.9.3.1 Uncertainty Band ... 71
2.9.3.2 Hysteresis ... 72
2.9.4 Unsaturated Soil Permeability ... 74
2.9.5 Theory of Moisture Flow ... 77
2.9.5.1 Saturated Flow ... 79
2.9.5.2 Unsaturated Flow ... 81
2.10 Numerical Methods ... 85
2.10.1 Numerical Methods Used in Solution of Richard’s Equation ... 86
2.10.2 Available Commercial Software ... 86
2.10.2.1 SVFlux ... 87
2.10.2.2 Vadose/W ... 88
2.10.2.3 Hydrus ... 89
2.11 Summary ... 89
3 CURRENT PRACTICE ... 94
3.1 Factors Affecting Residential Building Performance ... 94
3.2 Drainage Design Standards and Standard of Practice ... 97
3.3 Residential Foundation Design in USA ... 99
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3.5 Design and Construction Practice – Interviews with Industry ... 109
3.5.1 Geotechnical Engineering Interviews ... 109
3.5.1.1 Site Investigation and Soil Testing ... 109
3.5.1.2 Site Monitoring ... 111 3.5.1.3 Communication ... 111 3.5.1.4 Geotechnical Report ... 111 3.5.1.5 Design Procedure ... 112 3.5.1.6 Mitigation Measures ... 112 3.5.1.7 Areas of Problems ... 113 3.5.1.8 SWCC and Suction ... 113
3.5.2 Structural Engineering Interviews ... 114
3.5.2.1 Occurrence of Expansive Soils ... 114
3.5.2.2 Communication ... 114
3.5.2.3 Geotechnical Report ... 114
3.5.2.4 Structural Analysis and Design Procedure ... 114
3.5.2.5 Mitigation Measures ... 115
3.5.2.6 Areas of Problems and Concerns ... 115
3.5.3 Home Builder Interviews ... 116
3.5.3.1 Site Assessment ... 116
3.5.3.2 Budget and Design ... 117
3.5.3.3 Site Preparation Process ... 117
3.5.3.4 Site Monitoring ... 117 3.5.3.5 Communication ... 118 3.5.3.6 Mitigation Measures ... 120 3.5.3.7 Sources of Problems... 121 3.5.3.8 Litigation ... 121 3.5.4 Forensic Investigation ... 121
x 3.5.4.1 Failure Modes ... 121 3.5.4.1.1 Center Lift ... 122 3.5.4.1.2 Edge Lift ... 122 3.5.4.1.3 Settlement ... 122 3.5.4.2 Remediation Methods ... 123 3.6 Failure Criteria ... 123 3.7 Summary ... 125 4 LABORATORY DATA ... 127 4.1 Field Exploration ... 127 4.1.1 Equipment ... 128 4.1.2 Field Sampling ... 128
4.2 Soil Testing for Input Parameters ... 129
4.2.1 Moisture Content and Dry Density... 130
4.2.2 Atterberg Limits... 131
4.2.3 Sulfate Content ... 131
4.2.4 Cation Exchange Capacity ... 131
4.2.5 Specific Gravity ... 132
4.2.6 Expansion Index ... 132
4.2.6.1 Arizona Modified Expansion Index Procedure ... 132
4.2.6.2 Expansion Index Procedure as per ASTM D 4829 ... 133
4.2.7 Constant Volume Oedometer Testing ... 134
4.2.8 Consolidation Test and Correction Factors ... 135
4.2.9 Saturated Hydraulic Conductivity ... 137
4.2.10 Soil Suction ... 138
4.2.10.1 Pressure Plate ... 139
4.2.10.1.1 Equipment ... 139
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4.2.10.1.3 One Point Method of SWCC Determination ... 145
4.2.10.1.4 Complete SWCC ... 150
4.2.10.2 Filter Paper ... 151
4.2.10.3 Dessicator ... 153
4.2.11 Summary of Laboratory Results ... 153
4.2.11.1 Sampling Locations ... 153
4.2.11.2 Summary Tables ... 156
4.2.12 Selection of Input for Modeling ... 162
5 MAP OF EXPANSIVE SOIL DISTRIBUTION IN PHOENIX VALLEY ... 164
6 PTI RESIDENTIAL FOUNDATION DESIGN ... 171
6.1 Introduction ... 171
6.2 Historical Background ... 171
6.3 Definitions ... 173
6.4 PTI 2nd Edition Design Procedure, 1996... 174
6.5 PTI 3rd Edition Design Procedure, 2004 ... 176
6.5.1 Additional Definitions Provided in the Procedure. ... 177
6.5.2 Assumptions. ... 178
6.5.3 Procedure. ... 179
6.6 Design Parameters for Arizona ... 183
6.7 Discussion ... 184
6.8 Sensitivity Analysis ... 186
6.8.1 Influence of Suction Profiles on Geotechnical Parameters ... 187
6.8.2 Influence of Geotechnical Parameters on Slab Thickness ... 189
6.8.3 Sensitivity of ym to Suction Profile ... 190
6.8.4 Comparison of Different Suction Compression Index Methodologies ... 191
6.8.5 Influence of Gravel Correction ... 192
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7 MODELING – NUMERICAL METHODS ... 194
7.1 Modeling Challenges ... 194
7.2 Selection of Program ... 198
7.2.1 Convergence, Stability and Accuracy ... 198
7.2.2 Experiment Set-Up ... 202
7.2.3 Presentation of Results ... 203
7.2.4 Discussion and Conclusions ... 207
7.3 Sensitivity analysis of SWCC and k(h) ... 209
7.3.1 Uncertainty of Unsaturated Soil Functions ... 210
7.3.2 Problem Set-Up ... 211
7.3.2.1 Soil Properties ... 211
7.3.2.2 Initial and Boundary Conditions ... 213
7.3.2.3 Modeling Software, Mesh Size and Time Step ... 213
7.3.3 Numerical Simulation ... 213 7.3.3.1 Hysteresis in SWCC ... 214 7.3.3.2 Uncertainty in k(h) ... 215 7.3.3.2.1 Infiltration ... 215 7.3.3.2.2 Evaporation ... 217 7.3.4 Conclusions ... 218
7.4 SVFlux Program Behavior ... 219
7.4.1 Numerical Oscillations – Lessons Learned ... 220
7.4.2 Numerical Challenges ... 222
7.5 Numerical Experiments... 223
7.5.1 Fixed vs. Adaptive Time Step ... 223
7.5.2 Mixed Formulation ... 227
7.5.3 Normalization ... 228
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7.5.5 Time Discretization - Exponential Integrator ... 229
7.5.6 Time Discretization - ADI ... 229
7.6 Conclusions ... 230
8 MODELING – NUMERICAL RESULTS ... 231
8.1 Modeling Objective ... 231
8.2 Design of Experiment ... 232
8.2.1 Problem Assumptions and Restrictions ... 233
8.2.2 Program ... 234
8.2.3 SVFlux Specific Restrictions ... 235
8.2.4 Boundary and Initial Conditions ... 238
8.2.5 Domain Size ... 239
8.2.6 Soil Input Parameters ... 240
8.2.7 Determination of Appropriate Input Flux ... 243
8.2.7.1 Evaporation ... 243
8.2.7.2 Desert and Low Water Use Landscaping ... 248
8.2.7.2.1 Irrigation Needs of Desert and Low Water Use Landscape ... 248
8.2.7.2.2 Irrigation Systems ... 248
8.2.7.2.3 Input Flux for Desert and Low Water Use Landscape ... 249
8.2.7.2.4 Average Input Flux ... 250
8.2.7.3 Turf Landscaping ... 251
8.2.7.3.1 Irrigation Needs of Grass ... 251
8.2.7.3.2 Irrigation Systems ... 252
8.2.7.3.3 Typical Water Use on Turf Landscaping ... 252
8.2.7.3.4 Flux Input for Turf Landscaping ... 252
8.2.7.3.5 Average Input Flux ... 255
8.2.8 Output Presentation - Definitions... 256
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8.4 Simplification of Flux ... 263
8.4.1 Potential Evaporation ... 263
8.4.2 Precipitation and Irrigation ... 267
8.4.2.1 1-D Desert Landscape ... 267
8.4.2.2 1-D Turf Landscape ... 271
8.4.2.3 2-D Analysis of Average Absorbed Flux in Turf Landscape ... 275
8.4.3 Key Findings of Flux Simplification ... 279
8.5 Depth of Influence and Suction Variation with Depth ... 280
8.5.1 Desert Landscape – Dry IC ... 280
8.5.2 Desert Landscape – Wet IC ... 284
8.5.3 Desert Landscape - Ponding near Structure ... 286
8.5.4 Turf Landscape – Dry IC ... 290
8.5.5 Turf Landscape – Wet IC ... 296
8.5.6 Key Findings of 1D Analysis ... 297
8.6 Edge moisture Variation Distance Degree of Saturation ... 300
8.6.1 Desert Landscape... 300
8.6.2 Turf Landscape ... 300
8.7 Conclusions and Recommendations ... 304
9 FIELD EVIDENCE OF WETTING/DRYING INDUCED DAMAGE ... 308
9.1 Depth of Wetting and Depth of Active Zone ... 308
9.2 Forensic Investigations ... 322
9.2.1 Type of Data Collected ... 323
9.2.2 Sources of Suction Change Related Distress ... 324
9.2.3 Degree of Saturation and Suction Conditions below Foundations ... 333
9.2.4 Comparison of Landscape Type to Distress Magnitude... 337
9.2.5 Relative Slab Differential Data ... 338
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9.4 Key Findings ... 341
10 CONCLUSIONS AND RECOMMENDATIONS ... 344
10.1 Scope of Research Work ... 344
10.2 Conclusions ... 347
10.3 Future Research ... 351
REFERENCES ... 353
APPENDIX A HISTORY OF PTI GEOTECHNICAL PROCEDURE DEVELOPMENT ... 365
B LABORATORY DATA ... 379
C DETERMINATION OF SWCC USING ONE POINT SUCTION MEASUREMENT AND STANDARD CURVES ... 471
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Table Page
2.1. Angular Distortion Criteria Based on Design Manuals (summarized in Advanced
Foundation Repair, 2007). ...41
2.2. Newly constructed slab deviation from horizontal and angular distortion. ...42
2.3. Mineral clay properties (after Woodward-Clyde and Associates, 1967). ...53
2.4. Relation between swelling potential and PI (from Holtz and Gibbs, 1956). ...56
2.5. Expansive Soil Classification based on Atterberg Limits (Snethen et al.1977). ...56
2.6. Relationship between shrinkage and swell potential (after Altmeyer, 1955). ...58
2.7. Soil classification based on suction compression index (after McKeen, 2001). ...60
2.8. Classification of swell potential significance (after U.S. Bureau of Reclamation, 1974; surcharge of 6.9 kPa; Holtz et al., 1981). ...63
2.9. Classification of swell potential as per U.S. ASTM Standard D 4829-03 for Expansion Index. ...63
2.10. Proposed empirical and theoretical equations of SWCC. ...70
2.11. Proposed equations of unsaturated soil permeability as a function of suction (from Fredlund, 1993). ...75
3.1. Description of distress per Damage Category in AS2870. ...109
3.2. Description of distress per Damage Category in AS2870. ...109
3.3. Residential construction performance criteria in the first 2 years after homeowner occupancy (AROC, 2004). ...125
4.1. Main Equipment used for Field Sampling and Coring (after Perera, 2003). ...128
4.2. Classification of Potential Expansion based on EI (ASTM D 4829). ...134
4.3. RH and suction per saturated salt solutions at 20°C (based on Dean, 1999). ...153
4.4. Locations of Soil Sampling. ...154
4.5. Summary Table ...156
4.7. Average soil values. ...162
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6.1. Soil Index Properties Used in VOLFLO Input for Representative Soils ...185
6.2. Design Parameters for Representative Soils ...185
6.3. Design Parameters for All Soils from Chapter 4. ...186
6.4. Design Parameters for Sensitivity Study. ...187
6.5. PTI 3rd Edition Calculations for Example Profile for Various γh Methods ...191
7.1. Literature Review of Implemented Modeling Controls. ...196
7.2. Summary Table of Convergence Results ...208
7.3. SWCC parameters ...212
7.4. Summary of Modeled Scenarios ...214
7.5. Summary of numerical experiments, dx and dt. ...227
8.1. List of Performed Analyses. ...232
8.2. Soil Properties. ...243
8.3. Potential evaporation rate for Phoenix area, Arizona (from ADWR, NOAA and AMN 2006) and potential evapotranspiration rates for Bermuda turf landscape, Cave Creek, Arizona (UA, from Dep. of Agriculture, 2000). ...246
8.4. Landscape coefficients (from Dep. of Agriculture, 2005). ...247
8.5. Gallons of Water needed to Wet Root Zone per Irrigation Event (from City of Mesa, Department of Water Use, 2005). ...248
8.6. Average precipitation data from Phoenix Airport metrological station (from NCDC).249 8.7. Recommended irrigation pattern for warm season Bermuda grass (from City of Mesa, Department of Water Use, 2005). ...251
8.8. Amount of irrigation and potential evapotranspiration used in modeling of turf landscape. ...254
8.9. Average Input Flux for 2-D Analysis of CH Soil. ...255
8.10. Definitions of Input and Output Quantities. ...256
8.11. Mesh spacing, time step and run times for SM-ML analyses. ...262
xviii
8.13. HF to AF ratio of distance to 1000 kPa. ...278
8.14. Summary Table – Seasonal Depth of Influence; 1 Year Long Analysis. ...297
8.15. Summary Table – Seasonal Surface Suction; 1 Year Long Analysis. ...297
9.1. Saturation and Suction Variation with Depth for Undeveloped Desert. ...320
9.2. Saturation and Suction Variation with Depth for Agricultural Land. ...321
9.3. Residential Construction Distress Count vs. Landscape Type (distress beyond home owner responsibility defined by AROC). ...338
9.4. Frequency of slab mode deformation occurrence and average relative slab differential. ...340
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Figure Page
2.1. Schematic of water front movement (after McWhorter and Nelson, 1979). ...17
2.2. Idealized water content profile (after Nelson et al., 2001). ...18
2.3. Idealized suction profile of unsaturated soil (after Fredlund and Rahardjo, 1993). ....19
2.4. Slab movement, rainfall and site plan of experimental house at Vereeniging, Transvaal Highveld (after Blight, 1965). ...25
2.5. Soil Moisture Profile for soil a) under cover and without cover, b) difference in soil moisture profile between soil located below slab and outside of covered area (after Tucker and Poor, 1978). ...26
2.6. Center lift and edge lift slab distortion due to seasonal moisture variation (after PTI, 2004). ...27
2.7. Measured vertical ground movement within soil profile of Regina clay, Saskatchewan (after Hamilton, 1968). ...29
2.8. Typical maximum, minimum, and mean annual soil temperatures, 1959-1963 for a typical soil cross-section in Winnipeg, Manitoba (after Hamilton, 1969). ...31
2.9. Influence of evapotranspiration of trees on paved areas (after Snethen, 2001). ...34
2.10. Crack in residence wall due to vegetation (after Snethen, 2001). ...35
2.11. Sketch of crack and proximity of tree to the structure (after Snethen, 2001). ...35
2.12. Change in void ratio due to change in volumetric water content (after Nevels, 2001). ...38
2.13. Effect of initial dry density on swell and shrinkage (after Chen, 1988). ...38
2.14. Swelling and shrinkage behavior of expansive soils subject to repeated wetting and drying (after Chen, 1988). ...40
2.15. a) Bathtub effect of fill, b) Fat Clay cap and positive drainage to prevent the bathtub effect of fill (SlabWorks, 2008). ...47
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2.17. Relationship between repulsive forces of clay particles to half distance between particles for montmorillonite (after Philip Low). Similar relationship was developed
by Warkentine et al., (1957) for swell pressure vs. half distance. ...51
2.18. Relationship between percentage of swell and percentage of clay (after Seed et al., 1962). ...52
2.19. Mineralogical classification (after Pearring, 1963). ...54
2.20. Expansion potential based on cation exchange activity and soil activity (after Nelson and Miller, 1992). ...54
2.21. Mineralogical classification based on Atterberg Limits (Holtz and Kovacs, 1981). ...55
2.22. Soil swell potential in terms of activity and percent clay (Seed et al., 1962). ...57
2.23. Swell potential as a function of wPI (after Zapata et al., 2006). ...57
2.24. Expansive soil classification based on index soil properties (Holtz and Gibbs, 1956). ...58
2.25. Soil characterization in terms of suction compression index (after McKeen, 2001). ..60
2.26. Suction compression index based on mineralogical classification of soil into six types and soil index properties (after Covar and Lytton, 2001 and also PTI 3rd Edition). ...61
2.27. Typical Soil Water Characteristic Curve (after Fredlund and Rahardjo,1999). ...70
2.28. Uncertainty Band of Fountain Hills, Arizona clay (Zapata, 1999). ...72
2.29. Ink bottle effect (after Miyazaki, 1993). ...73
2.30. Closed and open hysteresis loops developed for CH soil, Arizona. ...73
2.31. Typical unsaturated permeability variation with volumetric water content. Comparison of empirical data to predicted values. (after Fredlund and Rahardjo, 1993). ...76
2.32. Schematic of flow classification based on Reynolds number (after Tindall and Kunkel, 1999). ...80
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3.2. Schematic of stem-and-footer. ...100 3.3. Cross-section of footing in PT slab (after 3rd edition PTI, 2004). ...101 3.4. Schematic of ribbed PT slab (after 3rd edition PTI, 2004). ...101 3.5. Schematic of uniform thickness PT slab (PTI, 1998). ...102 3.6. Schematic of uniform thickness PT slab (PTI, 1998). ...102 3.7. Schematic of raft foundation footing. ...103 3.8. Schematic of raft foundation (AS2780, 1996). ...103 4.1. Consolidation test on a steel plug; dummy specimen. ...136 4.2. Typical test results of constant volume oedometer test; correction to find swelling
pressure (after Fredlund and Rahardjo, 1993). ...137 4.3. Fredlund SWCC cell schematic (after Perera, 2003). ...139 4.4. Fredlund SWCC cell. ...140 4.5. Fredlund SWCC cell set-up (grooved platen not in the picture). ...141 4.6. Condensation on bottom plate inside of SWCC cell. ...143 4.7. Condensation on brass ring inside SWCC cell. ...143 4.8. Lateral soil shrinkage during SWCC testing. ...144 4.9. Soil cracking during SWCC test. ...145 4.10. Family of SWCC Curves for Plastic Soils Developed by Perera (Perera, 2003). ...146 4.11. Pressure plate and filter paper test results, SWCC estimate. ...149 4.12. Filter paper calibration curve. ...152 4.13. Sampling Locations superimposed on NRCS swell potential map. ...155 5.1. Natural Resources Conservation Service (NRCS) Swell Potential Map. ...164 5.2. ASTM D 4829 Expansion Index correlation with Arizona EI test (HBACA, 2006). ...166 5.3. Modified wPI vs. EIAZ relationship. ...167 5.4. Updated Swell Potential Map for Central Arizona, Phoenix Region in the Upper
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5.5. Updated Swell Potential Map for Central Arizona, Phoenix Region in the Upper 5-ft with few measured EIAZ data points. ...170 6.1. Edge Moisture Variation Distance as a Function of Thornthwaite Moisture Index
(after Wray, 1978). ...175 6.2. Variation of Soil Suction with Thornthwaite Moisture Index (PTI, 2004). ...179 6.3. Edge Moisture Variation Selection Chart (PTI, 2004) ...182 6.4. The ym sensitivity to LL. ...188 6.5. The ym sensitivity to PL. ...188 6.6. The ym sensitivity to % clay. ...189 6.7. The ym sensitivity to % clay. ...189 6.8. The ym sensitivity to % clay. ...190 6.9. Sensitivity Analysis of Gravel Correction Factor. ...192 7.1. Comparison of modeling results with different programs, Texas site, a) cumulative
AE and domain accumulation, b) relative errors (after Scanlon et al., 2002). ...201 7.2. Unsaturated soil properties a) SWCC and b) k(h)...203 7.3. Convergence Study for Hydrus, a) suction profile b) instantaneous flux. ...204 7.4. Convergence Study for SVFlux, a) suction profile b) instantaneous flux. ...205 7.5. Convergence Study for Vadose/W, a) suction profile b) instantaneous flux. ...206 7.6. Examples of stability issues in various software a) suction oscillation with depth, b)
actual flux oscillation at soil surface, and c) suction with depth increased
monotonically to unreasonable values. ...207 7.7. Software comparison a) Suction profile, and b) Instantaneous flux. ...209 7.8. Unsaturated soil properties; a) SWCC and b) Unsaturated soil permeability where
F1 is drying curve fitted though experimental data, F2 is wetting curve due to
xxiii
7.9. Influence of SWCC variation for the same k(h) obtained with F1 and p=12 and irrigation of 0.001 m/h. a) pore water pressure variation with depth, b) degree of saturation with depth and c) instantaneous actual flux. ...215 7.10. Influence of k(h) variation coupled with appropriate SWCCs and irrigation of 0.001
m/h. a) pore water pressure variation with depth, b) degree of saturation with
depth and c) instantaneous actual flux. ...216 7.11. Influence of k(h) variation coupled with appropriate SWCCs and PE of 0.0002 m/h.
m/h. a) pore water pressure variation with depth, b) degree of saturation with
depth and c) instantaneous actual flux. ...217 7.12. Input flux for numerical experiment. ...224 7.13. Implemented node spacing. ...225 7.14. Instantaneous flux and surface matric suction for adaptive and fixed dt
formulations. ...226 8.1. Analysis results: a) Input Flux, b) Net fluxes, and c) Matric suction at selected
depths. ...236 8.2. Analysis Results - Instantaneous flux. ...237 8.3. Boundary condition of control volume. ...239 8.4. SWCC – CH soil. ...241 8.5. Unsaturated Soil Permeability – CH soil. ...241 8.6. SWCC – SM-ML soil (after Pereira at al., 2005). ...242 8.7. Unsaturated Soil Permeability – SM-ML soil (after Pereira at al., 2005). ...242 8.8. Relationship between AE/PE to total suction for sand, silt and clay (after Wilson,
1997). ...245 8.9. PE for Phoenix area, Arizona (from ADWR, NOAA and AMN 2006) and PET rates
for tall, well watered grass and Bermuda turf landscapes, Cave Creek, Arizona (from Dep. of Agriculture, 2000). ...246 8.10. Suction as a function of RH and T. ...247
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8.11. Desert Landscape Flux. ...250 8.12. Turf Landscape Flux. ...254 8.13. Turf landscape, average absorbed flux per year for CH soil. ...256 8.14. Convergence analysis, January, PE only, desert landscape, SM-ML. ...260 8.15. Convergence analysis, January, precipitation, desert landscape, SM-ML. ...260 8.16. Convergence analysis, end of January, desert landscape, SM-ML. ...261 8.17. Components of PE for PE flux simplification analysis. ...264 8.18. Instantaneous and net AE for PE averaging analysis; CH soil. ...265 8.19. Suction at depth vs. time for PE averaging analysis; CH soil. ...266 8.20. Suction profile at the end of the PE flux averaging analysis; CH soil. ...266 8.21. Suction at depth vs. time for desert landscape analysis, a) CH, b) SM-ML. ...268 8.22. Suction profile at the end of analysis for desert landscape analysis, a) CH, b)
SM-ML. ...269 8.23. Instantaneous and cumulative flux for desert landscape analysis, a) CH, b)
SM-ML. ...270 8.24. Suction at depth vs. time for turf landscape analysis; a)CH, and b) SM-ML. ...272 8.25. Suction profile at the end of analysis for turf landscape analysis; a) CH, and b)
SM-ML. ...273 8.26. Instantaneous and cumulative flux for turf landscape analysis; a) CH, and b)
SM-ML. ...274 8.27. Instantaneous flux and domain accumulation for 2D turf landscape analysis with
HF and average absorbed flux obtained from 1D analysis. ...276 8.28. Variation of matric suction at the soil surface with time for 2D turf landscape
analysis; a) HF, and b) average absorbed flux from 1D analysis. ...277 8.29. Variation of suction with depth and time below the edge of the slab-on-grade for
xxv
8.30. Comparison of distance of influence to 1000kPa obtained with HF and average absorbed flux obtained from 1D analysis. ...279 8.31. Suction variation with depth and time, a) CH, b) SM-ML. ...281 8.32. Net flux per year for CH and SM-ML soils. ...282 8.33. Progression of wetting and drying fronts. ...282 8.34. Profile at wettest and driest conditions in year 6, a) CH, b) SM-ML. ...283 8.35. Progression of wetting front for CH soil due to rainfall. ...284 8.36. Suction variation with time and depth for CH soil, desert landscape with moist IC. .285 8.37. Profile at wettest and driest conditions for CH soil, desert landscape with moist IC.285 8.38. Suction variation with time and depth for CH soil zoomed in on precipitation in
December, desert landscape with moist IC. ...286 8.39. Suction variation with depth and time, a) CH, b) SM-ML. ...288 8.40. Profile at wettest and driest conditions, a) CH, b) SM-ML. ...289 8.41. SM-ML soil, plum like distribution of moisture with depth and time to maximum
depth of 1.8 m in November. ...290 8.42. Suction variation with depth and time for CH soil, a) surface detail in 3-D , b) 2-D
plot. ...291 8.43. Suction variation with depth and time for SM-ML...292 8.44. Depth of Influence for CH and SM-ML Soils. ...292 8.45. Depth of influence due to irrigation a) CH (year 1), b) SM-ML (year 1). ...293 8.46. Profile at wettest and driest conditions, a) CH (year 6), b) SM-ML (year 1). ...294 8.47. Profile at wettest and driest conditions, a) CH, b) SM-ML. ...295 8.48. Depth of influence due to irrigation for CH soil and moist IC. ...296 8.49. Monotonic Progression of Wetting Front. ...298 8.50: Suction variation at the soil surface for CH soil and desert landscape ...300 8.51. Suction variation at the soil surface for CH soil, 2D turf landscape, average flux
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8.52. Depth of influence: horizontal inwards the slab, vertical below the edge of slab and vertical 1-m away from the edge at landscaped conditions; 2D turf landscape,
average flux analysis. ...302 8.53. Distance of lateral moisture migration through soil below a slab. ...303 8.54. Suction variation at the soil surface for CH soil, 2D turf landscape, average flux
analysis. ...303 9.1. SWCCs and Equilibrium Conditions below Residential Foundation for Site #4;
Insitu, Undisturbed Soil Testing; Equilibrium Suction Identification Curve. ...310 9.2. SWCC dependence on dry density; Reconstructed Soil Testing on CL with LL=29,
PI=12, and P200=63%. ...310 9.3. Suction Range of the Suction Identification Curve. ...311 9.4. CH soil – Identification of Equilibrium Suction...313 9.5. SM soil – Identification of Equilibrium Suction ...314 9.6. CL soil – Identification of Equilibrium Suction ...316 9.7. SC soil – Identification of Equilibrium Suction ...318 9.8. Sources of structure distress – a) courtyard and b) concentrated roof runoff. ...325 9.9. Sources of structure distress – a corner the house creates with garage where
positive drainage away from structure is hard to maintain. ...326 9.10. Sources of structure distress – poor drainage, utilities in side yard, vegetation in
side yard, gutter discharge into side yard. ...326 9.11. Sources of structure distress – AC condensation discharge next to foundation. ...327 9.12. Sources of structure distress – soil erosion due to roof runoff. ...328 9.13. Sources of structure distress – soil erosion/undermining of low density soil below
homeowner installed flatwork ...329 9.14. Sources of structure distress – poor drainage. ...330 9.15. Sources of structure distress – poor drainage (positive slope), AC condensation
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9.16. Sources of structure distress – homeowner modified drainage and grading,
sidewalk blocks drainage, AC condensation discharge next to foundation. ...331 9.17. Sources of structure distress – homeowner modified drainage and grading,
sidewalk/pool blocks drainage, vegetation next to foundation. ...331 9.18. Sources of structure distress – homeowner modified drainage and grading,
vegetable garden is a source of water. ...332 9.19. Sources of structure distress – homeowner modified drainage and grading,
decorative boarder blocks drainage, sprinkler discharge next to foundation. ...332 9.20. Sources of structure distress – area of potential water ponding, sprinkler discharge
next to foundation. ...333 9.21. Degree of saturation below residential foundations at depth between 3’-5’. ...334 9.22. Measured saturation and suction variation below slab-on-grade for a) turf
landscape, b) desert landscape, c) mixed landscape or desert landscape with
areas of potential ponding. ...336 9.23. Equilibrium Suction below foundation. ...337 9.24. Potential Slab Shapes. ...339
xxviii
NOMENCLATURE Ac = Activity ratio,
AE = Actual soil evaporation mm
day , = af av
Fredlund and Xing SWCC fitting parameters, = Coefficient of compressibility m2 N , = bf Ca
Fredlund and Xing SWCC fitting parameters, =
++ Calcium,
CEAc = Cation Exchange Activity, CEC = Cation Exchange Capacity,
= cf
Ch
Fredlund and Xing SWCC fitting parameters, = Suction compression index,
=
C(h) Adjustment factor which forces the SWCC through zero water content at a suction of 106
Cs
kPa,
= Compression rebound curve Cv = Volumetric specific heat [J/(m3 Cv *°C)] = Coefficient of consolidation
cm
2s
, Cw = Climatic rating,Dmy = Vapor diffusion coefficient [m3 Dv
*s/kg],
= Diffusion coefficient of water vapor through soil [kg*m/(kN*s)], Dvap = Molecular diffusivity of water vapor in air [m2
em
/s]. = Center lift edge moisture variation distance,
e = Void ratio or
e1
exponent,
= Void ratio from the consolidation curve; the first point considered for the calculation,
xxix
e2 = Void ratio from the consolidation curve; the second point considered for the calculation,
EI = Expansion Index,
EIAZ = Expansion Index modified (Arizona), em = Moisture variation distance,
F = Percent of soil passing US sieve number 200, FF = Floor flatness,
Ff = Soil fabric factor,
FL = Local levelness,
g = Constant of gravitational acceleration
2 9.81 m s . Gs(coarse) = Specific gravity of solids,
h = u/ρ
H
g+y, total head equal to pressure plus elevation heads [m], =
+ Hydrogen,
H0 = Initial height of soil specimen [in],
H2dr = Average height of the specimen when the pressure is increased from σ1' to σ2' ; The value is divided by two for double drainage test [cm2
hr
],
= Total suction corresponding to the residual water content, θr [kPa], =
hr Hs
Fredlund and Xing SWCC fitting parameters, = Depth to constant suction,
∆H = Change in height of soil specimen [in], Δh = Height of water in a tube [m],
Change in total suction [pF],
∆H/L =
Ip
Hydraulic gradient = Instability index, Ipt = Instability index,
xxx k, ks, ksat = Saturated soil permeability [m/s], K = Absolute temperature [K],
=
k(ψ) Unsaturated soil permeability [m/h], k(h) = Unsaturated soil permeability [m/h], k(ψ)y = Unsaturated soil permeability [m/h],
K+ = Potassium,
K2SO4 = Potassium sulfate, KCl = Potassium chloride,
kunsat = Unsaturated hydraulic conductivity,
Lv = Latent heat of vaporization for water [J/kg],
m = Fitting parameter in van Genuchten permeability equation. m2w =
∂
θ
/
∂
u
≥
0
Mg
, slope of SWCC in Fredlund and Sing equation, =
++ Magnesium,
=
ms Mass of dried soil [g], =
mw n
Mass of water [g], = Porosity,
Fitting parameter in van Genuchten permeability equation.
Na+ = Sodium, NaCl = NH4 Sodium chloride, = + Ammonium,
P200 = Percent passing US sieve number 200, PE = Potential evaporation mm
day
, PIe = Effective plasticity index of the soil, R = Universal molar gas constant 8.314 J
mole K
°
,
xxxi
S = Source or sink in Richards’ equation [m/h], Degree of soil saturation [%],
Slope of the total suction in pF,
t = Time [h],
T = Time factor [T90 = 0.848], Absolute temperature [K] TMI = Thornthwaite Moisture Index,
t90 = The time it takes to reach 90% of primary consolidation due to applied load; the value is used when Taylor method is applied [s],
Δu = Change in total suction, ua = Pore air pressure [kPa],
Pore air pressure [101.3 kPa], ua – uw = Matric suction [kPa],
(ua – uw)b = Matric suction at air entry value [kPa],
uv = uvsRH, partial pore pressure due to water vapor [kPa], uw = Pore water pressure [kPa],
uv = Partial pressure of pore water vapor [kPa],
uvs = Saturation pressure of water vapor over a flat surface of pure water at the same temperature [kPa],
=
V Total volume [ft3
Vw
].
= Specific volume of water
kg m w 3 001 . 0 , 1 ρ ,
Domain Volume of Water;The volume of water retained in the entire domain at specified time. The output quantity is given per 1 m2 surface area.
xxxii
∆Vw = Domain Accumulation; The volume of water absorbed in or lost from the domain within analyzed time period calculated as the difference between the final and initial volume of water in the profile. The output quantity is given per 1 m2 surface area.
=
w Gravimetric water content [%],
=
w opt Optimum water content [%], =
wPI Weighted plasticity Index, Wv = Molecular mass of water vapor
kmol
kg
016
.
18
, Δ = Change in quantity, y = Elevation [m],ym = Differential center lift movement, Differential soil movement, Differential swell,
ys = Differential soil movement Zs = Active zone depth, α = Tortuosity factor of soil,
Diffusion coefficient,
α'swell/shrink = Modified unsaturated diffusion coefficient, α'weighted = Weighted modified soil diffusion coefficient, β = Crossectional area of soil available for vapor flow, λ = Thermal conductivity [W/(m*°C)],
Pore size distribution index, dimensionless,
σ = surface tension for air water interface [0.0073 N/m]; σ' = Effective stress applied to the sample [kPa],
σ1’ = Effective consolidation stress from the consolidation curve; the first point considered for the calculation,
xxxiii
σ2’ = Effective consolidation stress from the consolidation curve; the second point considered for the calculation,
Φ = Large relaxation constant γw = Unit weight of water [9.81 kN/m3]
=
γd Dry unit weight of soil [kN/m3] =
γd max γh
Maximum unit weight [pcf] = Suction Compression Index
γh swell = Swell suction compression index during swell. γmod = Modified suction compression index
γo = Suction compression index for 100% clay γt(wet) = Moist unit weight of soil
θ = Volumetric water content
θr = Residual volumetric water content, dimensionless, θs = Saturated volumetric water content or porosity of the soil;
=
θv Volumetric water content,
= ρd
ρw
Dry density [pcf] = Density of water
ψ = Matric suction [kPa],
Total soil suction in the soil [kPa]
ψ T = Total suction [kPa]
% = percent
%fc = percent fine clay
Θ =
θ
−
θ
rθ
s−
θ
rµm
, normalized water content, dimensionless, = micro meter
1.1 Overview
Problems associated with expansive soils are not widely appreciated outside the areas of their occurrence. The amount of damage caused by expansive soils has been estimated to exceed fifteen billion dollars annually. In years of extreme temperatures or rainfall the damage is most severe (Wray and Meyer, 2004). The structures commonly affected by expansive soils are residential structures, roads, irrigation canals and spillways. This report deals exclusively with the moisture flow through expansive soils in semi-arid regions in the context of slab-on-grade performance; however the theories presented and applications can be extended to other engineering structures, and other environmental conditions.
Through observation and research, it was well established that the damage to structures built on expansive soils is mainly caused by change in soil suction (water content) of soil that has shrink/swell potential. The potential change in water content is generally attributed to the environmental conditions, change in depth of water table, water uptake by vegetation, removal of vegetation or excessive irrigation of landscape. Due to those factors, slab-on-grade foundations must resist two types of expansive soil movement: short-term cyclic heave/shrinkage around the perimeter of the foundation and long-term progressive volume change beneath the center of the slab (Day, 1994). The slab can display three types of deformation, tilt, edge lift and edge drop. In the first scenario, one edge of the slab is higher relative to the opposite edge with a smooth transition between. Tilt is commonly observed in very stiff foundations where the soils below one side of the property heaved or shrank. In edge drop deformation, the edges of the foundation are lower relative to the center. Edge drop can be caused by number of mechanisms which are hard to identify without benchmarked surveys before and after movement; they include settlement, compression or shrinkage below the foundation perimeter or heave below the center of the foundation. Based on literature review, heave below the foundation might be caused by monotonic moisture migration or a pipe leak. Soil swell around the slab perimeter is commonly manifested by raised edges of the slab and is described as the edge lift condition.
Residential foundations are typically constructed on unsaturated soils; therefore the implementation of unsaturated soils mechanics into the slab-on-grade design is highly appropriate. Although the theory for the analysis of geotechnical problems involving unsaturated soils has been developed and has been known for the last four decades, and despite the well-recognized importance of suction, unsaturated soil mechanics is not widely implemented by practicing engineers. An investigation of practice throughout the Unites States showed that less than 20% of commercial geotechnical laboratories performed suction measurements on a regular basis (Zapata, 1999). This fact is attributed to several factors such as: 1) laboratory and field testing of unsaturated soils is perceived as costly, time consuming, and difficult to conduct, 2) large uncertainty associated with the direct measurement and/or prediction of the unsaturated soil properties (Zapata, 1999), 3) from a mathematical perspective the numerical solution of Richards’ equation, which describes the unsaturated moisture flow through soil is a very challenging problem characterized by stability and convergence difficulties, 4) numerical modeling of moisture flow through unsaturated soil with any of the available public domain or commercial software is non-trivial, and 5) the numerical solution may take a long time and requires the use of fast computers.
Due to the above-mentioned difficulties, the Arizona Homebuilder’s Association of Central Arizona (HBACA) sponsored a research program on expansive soils to 1) identify the depth and magnitude of wetting below residential foundations and under free field conditions, 2) identify factors associated with residential construction distress, and 3) assess foundation performance under various landscape schemes. A number of steps were taken towards the goal of developing a better understanding of expansive soils behavior and field conditions leading to problems with expansive soils. These steps included: 1) numerical modeling of moisture flow through expansive soils in one- and two-dimensions. Two extreme surface flux conditions were considered, desert and excessively irrigated turf landscapes. The numerical results are applicable to regions with low to moderate expansion potential and Phoenix, Arizona environmental conditions; 2) development of map illustration identifying location with low to
medium swell potential in the Phoenix Valley; 3) comparisons of the numerical results to field evidence on depth of wetting and depth of active zone, as well as to foundation field performance; 4) evaluation of stability, convergence and numerical challenges for modeling of unsaturated moisture flow through expansive soils using Richards’ equation. Sources of those instabilities were identified and potential numerical improvements discussed; 5) survey of Arizona region practitioners to identify current design and construction practices, and 6) statistical analysis of forensic investigations to identify the nature and common causes of residential distress.
1.2 Historical Background
The estimation of moisture flow through unsaturated soil for geotechnical engineering applications is a multifaceted problem involving a combination of empiricism and unsaturated soil mechanics theory. Due to the complexity of the problem and difficulties associated with the implementation, the industry has adopted a semi-empirical approach to the design and mitigation of foundations on expansive soils for residential dwellings. Many of these methodologies have been developed based on regional environmental conditions and therefore are applicable only to those specific regions of the world. Difficulties frequently arise when local experience is applied to different environmental conditions and soil properties. The literature review presented summarizes empirical findings relative to the moisture flow through unsaturated soil and the observed impact on lightly loaded structures.
Two types of slab systems are commonly used in residential construction, conventional stem-and-footer (with un-reinforced or lightly reinforced slab) and post-tensioned slabs. The design methodologies, in large part, are based on the anticipated post-construction change in the depth of wetting and degree of saturation. It is assumed that the soil suction of the undeveloped site comes to equilibrium with the existing environmental conditions at depth unaffected by seasonal climate variation. Thus the soil suction is referred to as the equilibrium. Based on literature findings, the active zone depth was estimated to vary between 1.2 m to 12 m
(4 feet to 39 feet), depending on the definition of the term and environmental conditions of test region (McKeen, 1980, 1981, 1985; O’Neill and Poormoayed, 1980; Thompson, 1992; Thompson and McKeen, 1995; Wray, 1989, 1997; Durkee, 2000, Chao et al., 2006).
An introduction of an impermeable cover at the soil surface, such as a slab-on-grade or a pavement, results in elimination of precipitation and reduction in potential evaporation (Day, 1994). Also, it is common for conditions in landscaped areas to change relative to pre-developed conditions. With time the suction within the soil profile comes to an equilibrium with the new environmental conditions. It is sometimes postulated that the suction below the slab is constant with depth and equal to the initial equilibrium suction (Nelson et al., 2001). Based on empirical evidence, the process of monotonic moisture migration due to capillary forces, moisture condensation below the slab and temperature gradients (Chen, 1988) occurs up to six years (Donaldson, 1965). Furthermore, it was observed that the 6-10 year long equilibration process is followed by a uniform relative reduction in heave (Donaldson, 1965), which might be related to fatigue of swelling. Fatigue of swelling refers to a decrease in a soil’s swelling potential as the drying-wetting cycles repeat. Chen (1988) illustrated that swell levels off at the fifth cycle when “relative equilibrium” is reached.
A long-term study of slab-on-grade behavior by Wray (1992) illustrated that short-term post-construction slab movement in arid regions is attributed to seasonal climate variation resulting in edge lift slab distortion. Continued monitoring revealed slow but increasing mound in the center of the slab, indicating that subsequently center lift distortion might occur if soils are not placed at the appropriate moisture content. On the other hand in humid regions, the short-term edge lift slab distress is quickly replaced with a center lift scenario, which is likely to occur due to edge drop (Wray, 1992).
An important parameter for slab design is the suction variation below the edges of the slab due to environmental or human imposed conditions. It has been postulated that the suction may vary 1) between liquid limit and shrinkage limit (conclusion based on measured gravimetric water content data of SM and CL soils below 10 000 slab-on-grades in Houston and San
Antonio, Texas, (Stryron et al., 2001)), 2) between 98 kPa and 9 800 kPa (McKeen, 2001), and 3) between 33 kPa to 3 300kPa in terms of total suction (PTI, 2004).
The edge moisture variation distance, em, defined as the distance over “which moisture will change due to wetting or drying influences around the perimeter of the foundation” (PTI, 2004), is difficult to measure experimentally. Few case studies include measured em values in arid regions. The existing data shows that em varies between 1.75 m (study of bike trail by Nevels, 2001) and more than 4.5 m (study of slab-on-grade where em exceeded half of the slab width Durkee, 2000). The em might approach the active zone depth (McKeen et al., 1990), although the PTI (2004) procedure limits the magnitude of em to 3 m (9 ft).
The slab-soil system performance is frequently evaluated in terms of slab relative deflection, angular distortion, or overall magnitude and extent of superstructure distress. Based on forensic engineering studies, cosmetic damage was correlated to 1.1-1.75” slab relative deflection and 1/300 angular distortion. Structural damage was found to occur at relative deflection larger than 3.5” and maximum angular distortion of 1/100 (Day, 1990, Skempton and MacDonald (1956), Marsh and Thoeny (1999)). The study of as-built floor levelness, however, suggests that these distress markers should be used with sound engineering judgement. Newly constructed slabs were found to exhibit on average 0.5” relative slab deflection and average angular distortion of 1/340. These values as-constructed values were found to be as large as 2.2” and 1/71 respectively, values corresponding to structural damage (Koenig, 1991, Marsh and Thoeny, 1999, Walsh, et al., 2001, Noorany et al., 2005).
Mitigation measures are employed to minimize potential soil movement and superstructure distress. They include 1) removal, replacement and recompaction, 2) chemical stabilization 3) passive moisture control with moisture barriers and 4) active moisture control. The economical feasibility of a mitigation measure depends on availability of material and expertise of mitigation team, as well as the timing of identification of the problem. In Arizona, active moisture control in the form of pad pre-wetting is the most commonly implemented mitigation method. The effectiveness of these methods remains to be quantified.
The literature review consensus message is that the depth of moisture migration, magnitude of suction variation with depth in open fields and below impermeable surfaces, the distance of horizontal moisture migration below a slab, and soil-slab system behavior (with or without mitigation measures) are highly dependent on 1) soil properties and 2) environmental and human imposed conditions around the edges of the structure. Geotechnical engineers are faced with the challenge of estimating design parameters for foundation system design purposes. In general, design guidelines provide a cookie cutter methodology developed based on a local experience in a geographic region, which may or may not be applicable to different soil and climatic conditions. When limited empirical data is available, numerical modeling of moisture flow through unsaturated soil can be performed to aid in selection of design parameters.
Numerical analysis of moisture flow through unsaturated soil involves implementation of unsaturated soil mechanics principles by solving Richards’ equation, a parabolic, stiff, advection-diffusion partial differential equation derived from mass conservation. Stability, convergence and time efficiency are problematic issues inherent to this class of problem. The currently implemented standard approach to solving the PDE follows a “method of lines”, also referred to as semi-discretization, where spatial derivatives are first approximated using a variety of (usually low order) finite difference or finite element schemes, and the resulting discrete system of ordinary differential equations (which also accounts for boundary conditions) is then solved using a time integrator. Three commonly used numerical software were reviewed: SVFlux, Vadose/W and Hydrus. It was concluded that numerical modeling of moisture flow through unsaturated soil is a very challenging and time consuming task, but it is helpful in identification of general moisture migration trends due to various soil and flux conditions.
1.3 Research Objective and Scope
The main objectives of this study on expansive soils include 1) identification of the depth and magnitude of wetting below residential foundations and under free field conditions, 2)
identification of local practice, 3) identification of factors associated with residential construction distress and 4) assessment of foundation performance under various landscape schemes. The study resulted in the following research contributions:
1. Identification of challenges in numerical modeling of surface flux and associated infiltration into unsaturated soils in arid regions.
2. Numerical modeling of infiltration into expansive soils for various landscape and surface water control schemes.
3. Surveys of practitioners to assess Phoenix region practices used in the design of residential foundation systems on expansive soils.
4. Development of an updated map of expansive soils distribution in the Phoenix region, commonly used by practitioners to assess soil properties in the preliminary analysis. 5. Evaluation of the PTI procedure for slab-on-grade foundations, for Arizona soils and
climatic conditions, the predominant methodology for current practice in Arizona.
6. Study of the suction profiles beneath slabs for equilibrium conditions, using direct suction determination and SWCC correlations.
7. Surveys of Phoenix area geotechnical firms to identify areas in the Phoenix Valley were forensic investigations thought to be linked to the presence of expansive soils have been conducted. This data was reviewed for determination of trends and soil expansion potential, as well as site landscape and draining conditions.
8. Assessment of numerical modeling results through comparison for consistency with forensic study findings and field data on depth and degree of saturation (suction).
1.4 Research Methodology
The research methodology can be divided into four major parts 1) laboratory testing, 2) numerical modeling, 3) data collection and 4) development of maps. Soil profiles from beneath sixteen (16) slabs were obtained for identification of soil index properties, variation of saturation and measured suction with depth. The selected sites were located in expansive soil regions in
the Phoenix metropolitan area as identified from the NRCS soil map (www.nrcs.usda.gov). The results obtained were used for 1) identification of input properties for modeling (soil properties and initial suction conditions), 2) identification of input properties for the PTI procedure to determine the range of potential results for Arizona soil and climatic conditions and 3) identification of suctions below foundations.
The first research objective, identification of the depth and magnitude of wetting below residential foundations and under free field conditions, and the fourth objective, assessment of foundation performance under various landscape schemes were satisfied, in part, through numerical modeling. A finite element program, SVFlux 5.80, was selected to model 1D and 2D moisture flow through two uniform unsaturated soil types, fat clay (PI=53) and silt (PI=12). Modeling was carried out to determine the degree of saturation, the horizontal and the vertical distance of moisture penetration under the slab using typical Arizona environmental and human imposed flux boundary conditions. The first flux scenario considered represents desert or low water use landscape. In this case, the irrigation was assumed to be negligible. The appropriate precipitation input was determined by performing statistical analysis of 24 years of precipitation data obtained from NCDC (www.ncdc.noaa.gov). It was found that average annual rainfall of 8 inches is typical for Arizona climatic conditions. Similarly, the potential evaporation of 91 inches per year was obtained from 1) US Weather Service, Arizona Department of Water Resources, 2) NOAA, Western Regional Climate Center, and 3) Arizona Meteorological Network. The second surface flux condition considered mimicked the typical watering pattern for turf landscaping, where the irrigation and precipitation provide about 101 inches of water annually, while the anticipated evapotraspiration is only about 46 inches annually (based on data published by University of Arizona, Dep. of Agriculture (2000).
In order to satisfy the second objective, identification of local practice, numerous geotechnical, structural and construction companies were interviewed. Additional firms were solicited for geotechnical and forensic data. The geotechnical data of soil saturation and index properties with depth were used to help complete the first objective. It was found that the
engineering community frequently uses the NRCS map in the preliminary assessment of soil properties. As part of this study, an updated map, incorporating the geotechnical data obtained in the survey was developed using ArcGIS.
The field data available from below foundations, free field undeveloped desert regions and agricultural land were compared with the general conclusions drawn from numerical modeling. Additionally, the forensic data of floor elevation differential, type and magnitude of structure distress, and landscape and drainage were used together with numerical modeling to satisfy objectives three and four, identification of factors associated with residential construction distress and assessment of foundation performance under various landscape schemes.
1.5 Outline of Report
The first chapter is used as an introduction and for organizational purposes. Objectives and methodologies are addressed. The main focus of Chapter 2 is literature review. The literature review includes a wide range of topics, whose understanding was necessary for the completion of the research objectives. It includes 1) factors affecting swell and moisture migration; 2) field observations of moisture flow and heave; 3) the soil response to changes in suction followed by 4) relative slab deviation from horizontal of newly constructed slab suggesting that the magnitude of structure distress cannot be identified from floor level survey alone, since the initial construction conditions are unknown; 5) commonly implemented mitigation measures; 6) classification of swell potential based on soil index properties. The presented ideas were used in the development of the PTI procedure. Also one of the correlations developed was used in updating the NRCS swell potential map; 7) introduction to unsaturated soil mechanics theory; 8) methods of matric suction measurement, and 9) numerical methods and available commercial software used in the solution of Richards’ equation.
The focus of Chapter 3 is residential construction failure criteria and current practice identified through survey of Phoenix, Arizona area geotechnical, structural and construction professionals located in Phoenix, Arizona. A brief overview of design methodologies
implemented in the USA and other countries is given. Based on the interviews, the Post-Tensioning Institute (PTI) slab-on-grade design procedure was identified as the most commonly implemented methodology in Arizona for the design and construction of residential slab-on-grade on expansive soils.
The soil profiles analysed are presented in Chapter 4, laboratory testing. The laboratory testing involved the collection of sixteen (16) soil profiles from below existing slabs-on-grade. The soils were tested for index properties, swell potential, saturated soil permeability and matric suction. The body of the Chapter 4 gives detailed descriptions of soil testing performed and a data summary. The detailed laboratory results are illustrated in Appendix B.
The interviews with industry revealed the significance on the Natural Resource Conservation Service, NRCS, swell potential map in the preliminary identification of soil properties. In Chapter 5 this map was updated using soil data supplied by practitioners and correlations developed in this study.
Also, based on the interviews with industry, it was identified that the 3rd edition PTI procedure is the most commonly implemented methodology for design and construction of residential slab-on-grade on expansive soil. Chapter 6 describes this methodology in great detail, along with presentation of a sensitivity analysis and a discussion of design values for Arizona soil and climatic conditions.
Chapter 7 discusses numerical challenges associated with the solution of Richards’ equation. From a mathematical perspective, Richards’ equation is an advection-diffusion partial differential equation (PDE) with stiff and parabolic characteristics. Equations in this class exhibit stability and convergence challenges, whose solution require specially developed stiff numerical solvers. This chapter discusses proper modeling techniques that a user of developed commercial or public domain software should implement. The chapter concludes with future research for implementation of more advanced solution methodologies developed by the mathematical community.
Modeling results for infiltration into expansive soils for various landscape and surface water control schemes are presented in Chapter 8. Two soil types were analysed, CH and SM-ML representing the range of typical soils found in the Phoenix Valley region that might exhibit shrink/swell. Results for 1-D and 2-D analysis are discussed, while details are presented in Appendix D.
The focus of Chapter 8 is the presentation and the analysis of field and forensic data. The field evidence on depth of wetting and active zone depth is provided, where the degree of saturation with depth versus undeveloped desert and previously agricultural land is given. The moisture/suction conditions below foundations, correlation of distress magnitude to landscape type, drainage and grading, analysis of differential slab differential and identification of factors contributing to residential construction distress are also discussed. Finally, the locations of forensic investigations were mapped and compared to the updated NRCS map, and the correlation between soil properties and forensic investigation incidence identified. Conclusions, recommendation, summary of findings, and future research are given in Chapter 10.
1.6 Key Findings
The following key findings were identified from this research study:
1. Richards’ equation is a stiff parabolic PDE whose solution requires the implementation of a stiff, implicit numerical solver. Methods typically implemented in software exhibit instabilities suggesting an implementation of pseudo-implicit solver. The instabilities are usually overcome by reducing mesh spacing, time step or both.
2. The solution variability due to the uncertainty of the unsaturated soil properties is large and potentially larger than the variability associated with different software selection. 3. Flux averaging can be successfully used in the analysis of moisture flow through soil
when flux is due to atmospheric conditions (no ponding), and when no runoff occurs. On the other hand, if runoff takes place, the flux averaging (e.i. over the period of a month) overestimates the depth of moisture influence and degree of saturation.
4. Desert landscape results in very shallow moisture migration soils common to the Arizona region; 5-cm due to precipitation; 0.5-m seasonal suction variation.
5. Edge moisture variation distance obtained from numerical modeling was limited to 10 cm under desert landscape conditions.
6. Turf landscape results in an increased wetting front (9-m after 34 years for CH soil and 7-m after 2 years for SM-ML soil) and very shallow depth of drying (2 cm). Short-term seasonal suction variations of 0.5 m for SM-ML soil and 1-m from CH soil was observed in the numerical analysis.
7. Edge moisture variation distance of 35 cm was observed for CH soil under turf landscape conditions for the conditions considered in numerical modeling. Monotonic moisture migration below the slab leveled off during 5th year at 2.2-m.
8. The critical scenarios with respect to foundation performance are 1) poor drainage resulting in 100% soil saturation up to the depth of 1-m. 2) initial moist conditions with desert landscape.
9. In general, the failure mode when comparing the 2nd Edition PTI procedure to 3rd edition is from center lift to edge lift, and increase in slab thickness.
10. In PTI procedure appears to overestimate volume change in extremely wet and extremely dry soil, but may give reasonable results in the intermediate range.
11. Suctions below foundations depend on landscape type. For turf landscape, the equilibrium suctions reach an average of about 500 kPa based on field measurements. Desert landscape leads to dry suction below the foundations with an average of 1500 kPa based on field data.
12. Problems associated with foundation performance are typically caused by improper drainage and grading.
13. Based on field evidence, for native desert profiles the average degree of saturation for SM, SC/CL, and CH is 30%, 40%, and 70% respectively; whereas for agricultural areas it is about 40%, 50% and 80% respectively within the upper 20'.
2.1 Introduction
Moisture flow through unsaturated soil is an extremely complex phenomenon consisting of fluid transport at micro-, meso-, and macroscales. The estimation of the phenomenon is further complicated by heterogeneity of soil medium, nonlinear unsaturated soil properties and volume change characteristics of moisture sensitive soils at various degrees of saturation. Although the first physically based formulation was introduced almost 100 years ago by Green and Ampt in 1911 followed by Richard’s continuity equation in 1931 the topic continues to be researched by engineers, soil scientists, hydrologists and mathematicians. The current research is focused on describing soil constitutive relationships, unsaturated soil properties, physical components of the flow phenomenon, mathematical algorithms and numerical methods needed to solve a form of Richard’s equation, a nonlinear, parabolic, partial differential equation to which analytical solution does not exist.
Simplified solutions to estimate the extent and degree of wetting are typically proposed by civil engineers with an eye on practical and economical approach for design of engineered structures. Implementation of these methodologies is especially important in arid or semi-arid regions and moisture sensitive soil sites, where the soil might experience shrinkage, expansion or collapse. Volume change is a response of moisture sensitive soil to a transient wetting process. It depends on soil properties, loading conditions imposed on the soil mass and flux conditions at the soil surface. The potential change in water content is generally attributed to environmental conditions, human imposed irrigation, influence of vegetation and accidental wetting due to broken pipelines.
An inadequate estimation of moisture flow has two types of economical impacts. On one hand, it cost more to build an over-designed structure and on the other hand it is expensive and inconvenient to fix and upgrade inadequately performing an under-designed structure. The amount of damage caused by expansive soils has been estimated to exceed two billion dollars annually. In years of extreme temperatures or rainfall the damage estimate reaches seven billion dollars per year (Chen, 1988). The structures commonly affected by expansive soils are
